Your guess is as good as ours, but most fans are leaning toward Death--which could be interesting considering Thanos's increased Marvel Universe profile (he's featured in a just-announced original graphic novel) around that same time.
So why would the Merc With the Mouth be getting married?
"The honest answer is that it was written into a Deadpool story, but more or less as a gag, and Jordan D. White, the editor, was like a dog with a bone," co-writer Gerry Duggan told Nerdist. "He was like, 'No, no, no – this is going to be a big deal.' You know, we wrote it, so it’s not like we weren’t behind it. The fun thing has been – after his misadventures, I guess you’d call them, in North Korea, in the story arc 'The Good, the Bad, and the Ugly,' he learned a lot about himself and why he is the way he is. He doesn’t really have too much of a life. Yes, this is a knee-jerk reaction from a wounded guy to go out there and sort of do something crazy to sort of jump start a life, but there is a real emotional reason for doing it, so I think that will be the part that will resonate. Then the actual gags of the wedding will be fun too, but there is a real grounding to it, and that’s what will make it fun to explore."
As to whether the marriage is a stunt that won't have much in the way of lasting ramifications, Duggan added, "As for how long she’ll be around, I’m not 100% sure, because when you get close to Deadpool, that’s maybe when I start to think about adding you in my deadpool."
The Death rumors seem to center around a couple of things--first of all, if you look closely at the bride's bouquet in the image above, it seems that the flowers make up a skull. Granted, it could be a Deadpool mask (earlier iterations of the puzzle, before they gave away the groom and thus the wedding's color scheme, had light pink flowers that looked much more skull-like)--but what appears to be teeth at the bottom make it feel more like a skull.
That's significant, of course, because while the bride is covered in flesh in the image above, her face is covered completely. Death in Marvel Comics is often depicted as fully skeletal, although sometimes she has a normal, human-like body with a skeletal face. In any event, the skull flowers could be a clue.
According to the Deadpool Wiki, Death "is in love with Deadpool because she cannot have his soul due to him not being able to die. Deadpool reciprocates his love for her and so tries to figure out ways for him to lose his healing ability, die and be with his beloved. Thanos, jealous because he understood Death preferred Deadpool's love over his, cursed Deadpool with immortality so they couldn't be together."
And that ties into the second reason people believe it might be Death; in the comics sequence at left, Deadpool assured her that he would find a way for them to be together. The image popped up on my Facebook feed moments after the announcement that it was Deadpool's wedding became official.
The idea that Death is the bride could also explain how some characters like Rogue and Scarlet Witch, who are presently dead, could appear in the wedding's audience and not be spoiling any major developments to take place between now and April in their own books.
Deadpool, of course, is a book that explores perception and reality, usually in a funny way, and that's meta enough that they could just write the whole storyline out of continuity easily enough. There's also the fact that Nightcrawler is presiding over the ceremony, even though his priesthood later turned out to have been a fantasy that never really happened.